The invention concerns a retrievable bridge plug.
In many situations it is necessary to isolate one or more zones in cased well. As an example, it may be necessary to isolate against fluid and pressure in an oil or gas well. In this situation, a bridge plug can be used to isolate against changes in pressure in both directions.
Such bridge plugs comprises in principle a sealing part for sealing the differential pressure, and an anchoring part for preventing movement of the bridge plug due to the pressure force. In oil and gas wells, the bridge plug will in many circumstances have to pass constrictions, for example valves and nipples (hereafter called "restrictions"), after which it becomes located in a wider casing diameter. Due to their constructions, known retrievable bridge plugs have a limitation in the expansion, which prevents use of bridge plugs in some oil and gas wells.
Known bridge plugs exist in many dimensions, adapted to the different casing dimensions where the plug is to be placed. This follows from the fact that conventional bridge plugs have a comparatively low expansion rate. The low expansion rate of conventional bridge plugs is partly due to the construction of the anchoring part, and partly due to the structure of the packing element. A common method for anchoring plugs has been to use conical slip segments which are forced out radially, between two conical pipes which are forced together axially. In this method, the expansion of the slip segments is limited by the outer diameter of the conical pipes. Without active pulling of the slip segments, they can become stuck in restrictions when being pulled out of the oil or gas well. The packing element expands when a rubber body is squeezed axially. At high pressure and great expansion, existing packing elements can creep after some time, which eventually will result in leakage over the packing element. When pulling existing bridge plugs, the elasticity of the rubber will see the packing element return to the shape it had before setting. Without active pulling of the packing element, a deformed packing element may lead to difficulties in pulling the bridge plug out of the well, because it can become stuck in restrictions.